DNA and Inheritance

We are built from DNA — it tells our cells what to do by providing the instructions to build proteins, which do most of the work in our bodies.

Genes and chromosomes

A gene is a section of DNA which codes for a protein. Proteins are like molecular ‘machines’ and come in a variety of forms, from antibodies to enzymes. Each protein has its own gene, which provide the cell with instructions on how to make it. The entire DNA within an organism is called its genome.

Genes are located on chromosomes within the nucleus.

DNA structure

A molecule of DNA is made up of two strands which are twisted around each other to form a double helix. The two strands are connected by bonds between specific bases. The bases present in DNA include adenine (A), thymine (T), guanine (G) and cytosine (C) where A always bonds to T and G always bonds to C. This happens because these pairs fit neatly together and we say their shape is complementary.

That means that if a DNA molecule consists of 20% guanine then it should also contain 20% cytosine, since there will always be equal amounts of the complementary base. We can then work out that the remaining 60% must be made up of the other two bases, adenine and thymine (30% of each).

Transcription and translation

For a gene to produce a protein, the DNA within the gene must first be copied into RNA in a process called transcription. This is important because DNA is too big and wrapped snuggly around chromosomes to leave the nucleus. Just like photocopying a single useful page out of a chunky library book, the cell makes an RNA copy of all the important information contained in the gene. RNA is similar to DNA, but has two important differences:

1. DNA is double-stranded whereas RNA is a single strand

2. RNA contains uracil in place of thymine

There are lots of different types of RNA which perform different functions. The type we are talking about here, which is used to transfer information from DNA for protein synthesis are called messenger RNA or mRNA. Once the mRNA molecule has been synthesised, it can leave the nucleus and enter the cytoplasm. From there it finds its way to structures called ribosomes, which are basically protein-building machines. The ribosome attaches itself to the RNA and slides along it. The ribosome ‘reads’ the mRNA in a series of three bases (such as AUG, CCA, GCU) called codons. Each codon corresponds to a particular amino acid. As the ribosome reads the codons, a transfer RNA (tRNA) molecule which has a complementary anticodon carries an amino acid to the ribosome. Once the ribosome has read through the length of the mRNA, a series of different amino acids will have been dropped off by several tRNA molecules. Bonds form between the amino acids to form a protein.

Alleles

Genes exist in alternative forms called alleles which give rise to differences in inherited characteristics. For example, the gene for eye colour exists in three variations (alleles) which code for blue eyes, green eyes and brown eyes. We possess two alleles for each gene, one on each chromosome.

The combination of alleles which we possess is called our genotype and the visible characteristics that these alleles produce is called our phenotype. For example, plants with the genotype Tt will have a tall phenotype and plants with genotype tt will have a short phenotype.

Just as a person with a dominant personality always seems to have the final word, a dominant allele ‘decides’ what characteristic will be expressed from the gene. The dominant allele is always expressed, even if you only have one copy. A recessive allele can be thought of as the ‘weaker’ allele and can also determine the expressed characteristic if the bossy dominant allele isn’t around. Recessive alleles require both copies to be present in order to be expressed. In the following example, B represents the dominant allele for brown eyes whereas b represents the recessive allele for blue eyes.

• BB (homozygous dominant) gives the brown eye phenotype

• Bb (heterozygous) gives the brown eye phenotype

• bb (homozygous recessive) gives the blue eye phenotype

Sometimes alleles can be codominant which are more harmonious, with both alleles contributing to the phenotype. An example is the tortoiseshell coat colour in some cats, resulting from a combination of B (codes for orange fur) and b (which codes for black fur). Inheriting both alleles results in a patchwork of black and orange, referred to as tortoiseshell.

In reality, it’s only a handful of characteristics which are coded for by a single gene like the examples we have seen above. Many phenotypic traits, such as height, are a result of the interaction of multiple genes. When several genes contribute to a phenotype, we call this polygenic inheritance.

Monohybrid inheritance

Monohybrid inheritance is the inheritance of one gene shown by a Punnett square. The parent gametes are written on the side, with the genotypes of possible offspring in the centre.

If the dominant R allele produces red flowers and the recessive r allele produces white flowers, then we can see that breeding two heterozygous individuals will produce offspring with a 3:1 ratio of red flowers to white flowers.

Pedigree Charts

Pedigree charts show how genetic characteristics are passed on through generations. They are used by medical professionals to determine the probability of a family member inheriting a genetic disease. Different symbols are used for male, female, healthy and diseased individuals. Horizontal lines show two unrelated individuals who have reproduced, with the vertical lines representing their offspring. In this pedigree chart, you can see that individuals 1 and 2 have had five children (individuals 3, 5, 7, 8 and 10).

We can use these charts to work out the genotypes of each individual. Since the affected father (individual 1) only passed on the disease to some of his children, we know that he must also have one healthy allele to pass onto his offspring, therefore he must be heterozygous. This means that the diseased allele must be dominant over the healthy allele, therefore all of the healthy individuals will have a homozygous recessive phenotype.

Sex chromosomes

The 23rd chromosome of a human sex cell predicts the gender of the baby. If the chromosomes are XY it will be a boy. If the chromosomes are XX then the baby will be a girl. If we place the genotypes for determining sex in a Punnett square, we can see that there is a 50% chance of the child being a boy or a girl (which makes sense really).

Mitosis and meiosis

Mitosis occurs when a cell makes an exact copy of itself, for growth and repair. We are constantly shedding skin cells and lose up to 40,000 cells each minute, making it important for our body to replace those cells with new ones. The process of mitosis produces two genetically identical diploid body cells, which means that they have a complete set of chromosomes (23 pairs or 46 in total).

Mitosis also occurs when an organism reproduces asexually. For instance, some plants can reproduce asexually by growing ‘off-shoots’ from the parent called ‘runners’. These off-shoots form due to the replication of cells by mitosis.

Meiosis produces four haploid non-identical sex cells, or gametes. The term haploid means they only have half the number of chromosomes as a typical cell (23 single chromosomes rather than 23 pairs). Two haploid gametes fuse to form a diploid fertilised egg during fertilisation.